Pulmonary drug delivery systems, commonly referred to medically as nebulizers, come in various forms. These, in turn can be broken down into various sub-groups. For example, the atomizer type of device uses the venturi principle of air passing across a pipe or orifice to draw liquid medication from a storage receptical and atomize it into small particles. Such devices can be operated by squeezing a bulb or with a pressurized container.
When ultrasonic energy of the right frequency and power is applied to a liquid, a very fine particle mist is released from the surface. At the frequency required to convert liquids, such as, water to a mist, the ultrasonic energy can be produced by electrically exciting a piezoelectric-material, such as, lead zirconate titonate, and mechanically coupling that material to the liquid. Of the total energy which enters a system of this type, some is converted to heat in the piezoelectric material, some may be converted to heat in the liquid, and the remainder is consumed at the liquid surface in the process of breaking away particles to form the mist.
In a medical application, this process is called nebulization and is used to convert medication to a mist for inhalation in the treatment of respiratory disease. In order to do this most effectively, the medication should be nebulized into particles or droplets of a particular size range and, as a general rule, the smaller the particles the better the penetration of the particles into the lungs and the bronchial passageways.
Earlier versions of ultrasonic nebulizers were intended for use primarily in the home or medical facility environment. However, miniaturization and the availability of small, highly efficient, rechargeable battery packs make it highly desirable to provide portable ultrasonic nebulizers which can be hand-carried and used as required in the treatment of respiratory disease.
In the past, ultrasonic nebulizers which have employed a piezoelectric material have encountered numerous problems, among which is the tendency of the material to rapidly degrade owing to the generation of heat, cavitation of the liquid caused by the high acoustic energy level, and chemical attack of the surface by medications and cleaning agents. Each time that acoustic energy crosses from one material to another, some is passed and some is reflected. Any material positioned between the transducer and the liquid for protecting the surface should possess high energy transmission efficiency and low energy reflection back to the transducer. It has been found that this condition can be created by providing a thin coating or plating of approximately 1/100 W, such as, teflon, polyimide or gold, or providing a cover having a thickness of W/2 or a multiple thereof, such as, W, 3W/2, 2W attached to the transducer surface where .cent.W.infin. is the wavelength of the excitation signal. Glass is a preferred material for such a cover because it presents an easily cleaned and durable surface to the liquid and can tolerate high temperatures. Nevertheless, a coupling agent is required to bridge the air gap between the two surfaces. In U.S. Pat. No. 4,109,863 to Olson et al, it was proposed to employ adhesives for this purpose. However, high temperatures tend to weaken the bond of the adhesive and cause poor acoustic coupling and increased reflected energy. Olson et al proposed to solve the problem of high temperatures at the transducer surface by circulating a cooling water over the transducer and glass, but this method is not feasible for a portable handheld device and has the additional undesirable effect of acoustically damping the back side of the transducer and thus reducing the efficiency of the nebulizer system.
We have found it desirable to employ oil of the correct viscosity and temperature capability as a coupling agent. The oil tends to migrate toward the high energy density center of the transducer/glass interface and occurs even after high temperatures have forced some of the oil to the periphery. In order to overcome any tendency of the oil film to be too thin, causing reduced nebulization, the gap between the protective cover and the transducer surface must be so shaped as to provide an optimum oil film thickness thereacross which will avoid regularly generated reflections. Also it is important to contain the oil so that gravity and capillary forces do not carry it away from the gap during periods of inactivity. Accordingly it is important that the oil be confined or sealed in such a way as to assure that it will migrate towards the center of the gap when energy is applied. Moreover, another problem associated with the use of oil as a coupling agent is the presence of entrapped gas which, when released during operation, may displace the oil and uncouple the glass cover. It is therefore desirable to minimize the amount of entrapped gas present in the oil in the process of assembling the elements of the nebulizer and to make provision for accumulation of any entrapped gas which may escape from the transducer surface during its life.
Among various other prior art techniques, U.S. Pat. No. 3,433,461 to Scarpa employs a piezoelectric crystal bonded to a support layer. Both the crystal and the support layer were chosen to be one-half wavelength in thickness and bonded together with an adhesive to form a composite structure one wavelength in thickness. This structure is supported around its periphery and contains a thin web between the vibrating center and supported periphery to prevent support structure loading which is counter productive to efficient high energy vibration. Other patents of interest are U.S. Pat. No. 4,094,317 to Wasnich and U.S. Pat. No. 3,861,386 to Harris where an acoustic wave form is shaped to perform ultrasonic nebulization of the liquid and to isolate the liquid from the piezoelectric crystal and prevent dry operation of the device.